A. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus capable of generating ultrasound images by applying a pulse inversion technique, and more particularly, to an ultrasound diagnosis apparatus for generating ultrasound image data based on the nonlinearity characteristics of harmonic wave components of echo receiving signals due, for example, to a contrast agent introduced into an object.
B. Background of the Invention
An ultrasound diagnosis system transmits ultrasound pulses from ultrasound transducers (hereinafter “transducers”) installed in a head portion of the ultrasound probe to an object, such as a patient. The transducers receive reflected (echo) ultrasounds that are generated in accordance with differences of acoustic impedances of organs in the object in order to display the organ images on a monitor. Since an ultrasound image diagnosis apparatus can easily obtain and display a two dimensional image or a three dimensional image of B mode data or color Doppler data in real time by simply touching an ultrasound probe to a patient's body surface, it is widely used as an apparatus for diagnosing the status of a target organ in a patient's body.
In recent years, an ultrasound contrast agent suitable for using the ultrasound diagnosis of a circulatory organ region, such as a heart or an abdominal region has been developed. The ultrasound contrast agent (hereinafter, simply referred to as a “contrast agent”) includes micro-bubbles having low invasion characteristics to an object. By injecting this contrast agent into a heart or blood vessels in an object, the ultrasound examinations in cardiac organ region or abdominal region can be performed. This technique is referred to conventionally as the contrast echo method. The contrast echo method can accurately observe the blood flow statuses in the abdominal regions where blood flow speed is extremely slow. Consequently, due to the slow blood speed, the conventional color Doppler method can not be readily utilized for the ultrasound examinations in the abdominal region. Accordingly, the contrast echo technique is expected to improve the accuracy of the ultrasound diagnosis for tumor tissues of feeble blood flow amounts.
In the ultrasound examination by using the contrast agent, micro-bubbles of a contrast agent injected into blood vessels become strong ultrasound reflection sources. Consequently, it becomes possible to effectively observe feeble tissue blood flow data by detecting reflection waves from the contrast agent that moves in accompany with blood flows. However, a problem occurs when relatively strong ultrasounds are irradiated to the micro-bubbles in order to acquire image data having a good S/N ratio. In this case, the reflection intensity of the contrast agent is remarkably reduced due to breaks of the micro-bubbles. This is a severe problem.
In consideration of such characteristics of the contrast agent, a technique for performing a first ultrasound transmission/reception and a second ultrasound transmission/reception to the same region of an object at a prescribed time interval by using strong ultrasounds emitting to the same region of an object after dosing a contrast agent has been developed. In this technique, the reflection wave from the contrast agent is extracted by performing a subtraction between the reception signals acquired through the first ultrasound transmission/reception and the reception signals acquired through the second ultrasound transmission/reception to the crushed region of the micro-bubbles due to the first ultrasound transmission/reception (for instance, see Publication Japanese Patent Application Publication H8-336527).
Meanwhile, when transmitting ultrasounds are irradiated into the micro-bubbles, relatively larger harmonic wave components are generated due to the acoustic nonlinearity of the micro-bubbles. A polarity of the waveform formed by the harmonic wave components does not depend on the polarity of the transmitting ultrasounds. By using such characteristics, a more recent ultrasound diagnosis imaging technique has been developed. The technique is referred to as the pulse inversion method (PI method) (for instance, see U.S. Pat. No. 6,095,980). According to the PI method, ultrasound transmissions/receptions are performed twice onto the same region in an object at a prescribed interval by using a first and a second transmitting ultrasounds that have the same amplitude, both being small enough to avoid crushing of the micro-bubbles, and the first and second transmitting ultrasounds having a mutually inversed phase, i.e., different in or separated in phase by 180 degrees. The PI method further extracts the harmonic wave components in the reception signals due to the micro-bubbles in the contrast agent by performing a summation of the reception signals acquired through the first ultrasound transmission/reception and the reception signals acquired through the second ultrasound transmission/reception.
Further, another technique for simultaneously observing blood flowing data and organs data has been proposed by composing the image data based on the harmonic wave components of the reception signals acquired through this PI method and applying the maximum value maintaining calculation method to the image data based on the fundamental-wave component of the reception signals (for instance, see Publication Japanese Patent Application Publication 2007-236738).
According to the above-mentioned techniques, since the harmonic wave components included in the reception signals can be extracted, it becomes possible to grasp blood flowing data in blood vessels by observing the movement of the contrast agent that is the main generation source of the harmonic wave component. For example, it can differentiate normal tissues in which a lot of contrast agent exists in the bloods therein from tumor tissues in which a small amount of the contrast agent exists by extracting the tumor tissues of an ischemia status, i.e., the reduced blood flow status in the tumor tissue shows a reduced contrast from the harmonic wave components of the reception signals acquired as compared to each of the normal tissues existing around the tumor tissue having higher blood flows.
However, since living body tissues also have acoustic nonlinearity similar to the contrast agent, the reception signals acquired from the living body tissue also include the harmonic wave components by irradiating the ultrasounds on the living body tissues. In particular, the reception signals acquired from the tumor tissues usually include substantial harmonic wave components. Accordingly, when the tissues where a large amount of the contrast agent exists are differentiated from the tissues where a small amount of the contrast agent exists by extracting the harmonic wave components of the reception signals acquired from living body tissues dosed with the contrast agent, it become impossible to accurately delineate the contrast agent data flowing into the tissues caused by the mixing of the harmonic wave components occurred due to the nonlinearity of living body tissues into the harmonic wave components caused by the nonlinearity of the contrast agent.